1. Field of the Invention
The present invention relates to a power-driven screwdriver for successively driving screws into a workpiece.
2. Prior Art
A known screw tightening apparatus as disclosed in Japanese Laid-Open Patent Publication No. 53-37968, published Apr. 7, 1978, includes a screw indexing lever movable to advance a strip of screws when a fork-shaped protective plate or a leg plate is forcibly pressed against a workpiece. The strip of screws is subjected to undue tensile forces which tend to skew a screw in a position to be driven. The prior apparatus also has a slide block and a slide plate which are normally urged by a spring toward a position in which the free end of a screwdriver bit is disposed in the slide block, and the spring is displaced off the axis of the screwdriver bit. When the slide block and the slide plate are forced to slide into an outer frame against the resiliency of the spring before a screw is driven by the screwdriver bit, the slide block and the slide plate are liable to become inclined with respect to the screwdriver bit, a disadvantage which renders the screwdriver apparatus difficult to handle. The screw indexing lever is also spring-loaded to feed screws, and the operator finds the known screw tightening apparatus quite heavy and sluggish during screw driving operation. The conventional power-driven screwdriver is also disadvantageous in that it can easily damage the workpiece, the worn screwdriver bit cannot readily be replaced with a new one, and the screw being driven is likely to get tilted at it passes through the fork-shaped protective plate.